Three-way conversion catalysts and methods for the preparation therefor

ABSTRACT

A platinum group metal three-way conversion catalyst composition containing a high temperature catalytic component and a low temperature catalytic component has each catalytic component present as separate distinct particles in the same washcoat layer. The catalyst composition is prepared from a washcoat slurry containing a high temperature catalyst support material, and a low temperature catalyst support material, each support material being of sufficiently large particle size so as to prevent each support material forming a solution or a sol with the liquid medium of the slurry. The platinum group metal or metals can be impregnated into each support material either after formation of the washcoat on a non-porous refractory, metallic or palletized substrate or before forming the washcoat slurry.

This application is the U.S. national-phase application of PCTInternational Application No. PCT/GB97/01944.

This invention relates to catalysts used to remove undesirablecomponents in the exhaust gas from internal combustion engines. Moreparticularly, the invention is concerned with improved catalysts of thetype generally referred to as three-way conversion or TWC catalysts.

The exhaust from internal combustion engines contains hydrocarbons.carbon monoxide and nitrogen oxides which must be removed to levelsestablished by various government regulations. The aforementionedthree-way catalysts are poly-functional in that they have the capabilityof substantially simultaneously catalysing the oxidation of hydrocarbonsand carbon monoxide and the reduction of nitrogen oxides.

Typical three-way catalysts which exhibit good catalytic activity andlong life contain one or more platinum group metals (eg Pt, Pd, Rh, Ruand Ir) located upon a high surface area porous refractory oxidesupport, eg a high surface area alumina coating. The porous refractoryoxide support is carried on a suitable non-porous refractory substratesuch as a monolithic carrier comprising a refractory ceramic or metalhoneycomb structure or refractory particles such as spheres, pellets orshort extruded segments of a suitable refractory material.

Three-way catalysts are currently formulated with complex washcoatcompositions containing stabilised alumina, an oxygen storage component(primarily stabilised ceria) and precious metal catalytic components.The term “oxygen storage component” is used to designate a materialwhich is capable of being oxidised during oxygen-rich (lean) cycles ofthe exhaust gas being treated and reduced during oxygen-poor (rich)cycles of the exhaust gas being treated.

Three-way catalysts typically have been based on platinum/rhodiumcatalysts in preference to palladium which suffered from certaindisadvantages including the high sensitivity of palladium to poisoningby sulphur and lead. However, with increased use of lead free petrolaround the world, palladium is an extremely promising substitute for thetraditionally used platinum/rhodium catalysts. Furthermore, the muchlower cost of palladium makes it a highly desirable alternative toplatinum/rhodium in three-way catalysts, provided the desired catalyticperformance can be achieved.

The art has devoted a great deal of effort in attempts to improve theefficiency of palladium containing three-way catalysts. Thus, in anarticle in Third Int. Cong. Catal. and Auto Poll. Controls, Pre-printVol. 1, pages 125 to 135, the authors, Dettling et al, describe theinclusion of a low temperature catalyst component (Pd/Al₂O₃) and a hightemperature catalyst component (Pd/CeO)₂ in the same catalystcomposition for high activity under both low and high operatingtemperatures.

WO 95/00235 (Engelhard Corporation) also describes a palladiumcontaining catalyst composition containing low and high temperaturecatalyst components structured as two washcoat layers

WO 95/07600 (Allied Signal) describes a palladium containing three-waycatalyst as a single layer. However, according to the method ofpreparation, the finished catalyst only has the high temperature Pd/CeO₂component.

U.S. Pat. Nos 4,727,052, 5,057,483, 5,008,090 and 5,010,051; GB Patent1495637; and European Patent Applications 92302928.4 and 0427293A2 alsodescribe three-way conversion catalysts based on platinum group metalcatalytic components.

We have found that platinum group metal three-way catalysts containing ahigh temperature functional component and a low temperature functionalcomponent when prepared by the unique methods of the present inventionexhibit greatly improved three-way catalytic activity even afterextended high temperature aging.

In this specification, by high temperature functional catalyticcomponent is meant a catalytic component which exhibits catalyticactivity at higher temperatures (eg above about 500° C.) and by lowtemperature functional catalytic component is meant a catalyticcomponent which exhibits catalytic activity at lower temperatures (eg inthe range 200 to 400° C.)

According to the present invention there is provided a method of makinga platinum group metal three-way catalyst composition which contains ahigh temperature catalytic component and a low temperature catalyticcomponent with each catalytic component being present in the catalystcomposition as separate distinct particles in the same washcoat layer,which method comprises:

(a) forming on a non-porous substrate a combined washcoat of a hightemperature catalyst support material and a low temperature catalystsupport material from a slurry in which each of the catalyst supportmaterials is of sufficiently large particle size so as to prevent eachcatalyst support material from forming a solution or a sol with theliquid medium of the slurry; and

(b) impregnating a platinum group metal or metals into each catalystsupport material either after formation of the washcoat on thenon-porous substrate or before forming the washcoat slurry.

Preferably, separate slurries of the high temperature support materialand the low temperature support material are prepared and the twoslurries are then blended together and coated onto the non-poroussubstrate.

The non-porous substrate may be a refractory ceramic or metal honeycombstructure or refractory particles such as spheres, pellets or shortextruded segments of a suitable refractory material.

Further according to the present invention, the proportions of the hightemperature catalytic component and the low temperature catalyticcomponent required in the catalyst composition are determined by therespective water absorption capabilities of each catalyst supportmaterial and the respective amounts of each catalyst support materialpresent in the washcoat.

Preferably, the water absorption capabilities of the high temperaturecatalyst support material and the low temperature catalyst supportmaterial are respectively 0.2 to 1.0 ml/g and 0.5 to 2.5 ml/g.

Suitably, the catalyst support materials have a mean particle size ofless than 20 microns, preferably between 1 and 20 microns and morepreferably about 5 microns.

The platinum group metal is selected from platinum, palladium, rhodium,ruthenium, iridium or any combination thereof.

Preferably, the high temperature catalyst support material is an oxygenstorage material.

Suitable oxygen storage materials include ceria, perovskites. NiO, MnO₂and Pr₂O₃ with stablised ceria being the preferred material.

Suitable stabilisers for ceria include zirconium, lanthanum, alumina,yttrium, praeseodymium and neodymium with zirconium being preferred.

Suitably, the zirconium stablised ceria contains 2 to 50% ZrO₂, apreferred composition being about 58% by weight CeO₂ and about 42% byweight ZrO₂.

Suitable low temperature catalyst support materials are stabilisedalumina and unstabilised alumina.

Suitable stabilisers for alumina include lanthanum, barium and zirconiumwith lanthanum being preferred.

Preferably, the lanthanum stabilised alumina contains 2 to 7% lanthanumoxide.

The method of the invention may utilise a catalyst promoter, preferablyselected from Nd, Ba, Ce, La, Pr, Mg, Ca and Sr with Nd and Ba beingparticularly suitable. The catalyst promoters may be added to the slurryor separately impregnated.

Further preferably, the method of the invention utilises a compoundeffective for the suppression of hydrogen sulphide emissions from thecatalyst composition. Suitable such compounds include NiO, Fe₂O₃ and BaOwith NiO being preferred.

Suitably, the method according to the invention utilises a compoundwhich is effective in preventing preferential absorption of the platinumgroup metal in one or other of the high temperature or low temperaturecatalyst support materials. Preferred such compounds include citricacid, acetic acid and oxalic acid.

From another aspect, the present invention is a platinum group metalthree-way catalyst composition made by any of the methods describedabove.

From yet another aspect the present invention is a platinum group metalthree-way catalyst composition comprising a high temperature catalyticcomponent and a low temperature catalytic component wherein eachcatalytic component is present in the catalyst composition as separatedistinct particles in the same washcoat layer.

Suitably, the high temperature and low temperature catalytic componentsin the catalyst composition have a mean particle size of less than 20microns, preferably between 1 and 20 microns and more preferably about 5microns.

As can be seen from the foregoing discussion of the prior art, theconcept of combining a high temperature catalytic component and a lowtemperature catalytic component in the same three-way conversioncatalyst is known. The present invention however, enables both catalyticcomponents to be advantageously incorporated into a single washcoatlayer by utilising a unique preparation technique. This preparationtechnique entails incorporating two distinct and separate catalystsupport materials into the same washcoat slurry so that the finalcatalyst composition has both the high temperature catalytic functionand the low temperature catalytic function in a single washcoat layer.

A key feature of the invention is that the catalyst support materialsshould not be in solution in the washcoat slurry or present as verysmall particles as found in a sol (the order of magnitude of the size ofsol particles being in the nanometer range). In order to obtain thebenefits of the present invention, the insoluble catalyst supportmaterials in the washcoat slurry preferably should have a mean particlesize of at least 1 micron, more preferably about 5 microns. However, ifthe particle size is too large (eg greater than 20 microns) there may bedifficulty in getting the washcoat to adhere to a non-porous substrate.

Another important feature of the invention is that to maintainseparation of the catalyst support materials they should be ball-milledin separate slurries followed by blending of these slurries. The finalblend is coated onto the non-porous substrate.

Yet another important feature of the invention is the incipient wetnesswater absorption capabilities of the high temperature catalyst supportmaterial and the low temperature catalyst support material because thesewater absorption capabilities relate not only to the process for makingthe catalyst composition but also to the specification of the catalystformulation. The catalyst contains two oxide support materials,exemplified by zirconium-stablilised ceria and lanthanum-stabilisedalumina, although unstabilised alumina may be used. The platinum groupmetal (exemplified by palladium) is split between the two oxide supportmaterials. In one embodiment of the invention, the palladium isimpregnated from an aqueous solution into the washcoat consisting of amixture of the two oxide support materials and the way in which thepalladium is split between the two oxides depends on the fraction of theaqueous impregnation solution absorbed by the respective oxides. Forexample, if it is required that 50% of the available palladium is to besupported on the zirconium-stabilised ceria and the other 50% ofavailable palladium is to be supported on the lanthanum-stabilisedalumina then the washcoat would be formulated so that the waterabsorption of the zirconium-stabilised ceria in the catalyst composition(ie (ml water absorbed/g)×(g in catalyst)) is equal to the waterabsorption of the lanthanum-stabilised alumina in the catalystcomposition. Thus, the ratio of the oxide support materials is specifiedby their relative water absorptions and the absolute amounts of theoxide support materials is specified by the amount of support needed inthe catalyst composition (more specifically, a certain amount ofZr-stabilised ceria is needed for adequate performance). The desiredsplit of the palladium depends on the duty required of the catalystcomposition. In some applications, equal amounts of high temperaturecatalytic component and low temperature catalytic component is required.In other applications, more high temperature compound than lowtemperature compound is required (or vice versa). For example, catalystcompositions having palladium splits ranging from (a) 27% of Pd asPd/ZrCeO₂—73% of Pd as Pd/La Al₂O₃ to (b) 73% of Pd as Pd/ZrCeO₂—27% ofPd as Pd/La Al₂O₃ have been prepared according to the methods of theinvention.

In an alternative method of making the catalyst composition, a portionof the total palladium is impregnated into a bulk form of the hightemperature catalyst support material and the remaining portion of thepalladium is impregnated into a bulk form of the low temperaturecatalyst support material prior to the formation of the washcoat slurry.Since the impregnated palladium is essentially insoluble in the washcoatit remains interacted with its associated oxide support material in thefinal catalyst composition. In this embodiment also, the ratio of thetwo oxide support materials is chosen on the basis of their relativewater absorptions and the desired split between the palladium intimatelyinteractive with, for example, the zirconium stabilised ceria and thepalladium intimately interactive with the lanthaniun stabilised alumina.

Certain embodiments of the invention and the efficacy thereof aredemonstrated by the following Examples.

EXAMPLE 1

La-stabilised Al₂O₃ with an incipient wetness water absorption of about1.85 ml/g was slurried in water at a composition of about 55% by weightsolids to form slurry A. Slurry A was then wet milled to a mean particlesize of about 5 microns. Separately, bulk NiO was slurried in water at acomposition of about 4% by weight solids and wet milled to a meanparticle size of about 5 microns. After the NiO slurry was wet milled.Zr-stabilised ceria with an incipient wetness water absorption of about0.5 ml/g and was added to the NiO slurry and the resulting slurry waswet milled further to a mean particle size of about 5 microns to formslurry B. Slurry A and slurry B were blended in a specific ratio (1:2.36as dictated by the desired catalyst composition) and coated on amonolithic cordierite substrate by dipping or alternatively passingthrough a washcoat curtain. After blowing off the excess washcoat withcompressed air, the coated substrate was then dried at 60° C. and firedat 500° C. in flowing air. The fired coated substrate was then dipped inan aqueous solution of Pd(NO₃)₂/citric acid/Nd(NO₃)₃ or alternativelypassed through a curtain of the same solution and excess solution wasblown off with compressed air (Impregnation 1). The Pd(NO₃)₂/citricacid/Nd(NO₃)₃ Solution was absorbed such that the quantity of solutionwhich just filled the pores of the washcoat contained sufficient Pd andNd to give the desired loadings. The resulting impregnated block wasdried at 60° C. and fired at 500° C. in flowing air. Finally, the firedblock was dipped in an aqueous solution of barium acetate (about 150 gBa/l) or alternatively passed through a curtain of the same solution andthe excess solution was blown off with compressed air (Impregnation 2).The barium acetate solution was absorbed such that the quantity ofsolution which just filled the washcoat contained sufficient barium togive the desired loading. Finally, the barium-impregnated block wasdried at 60° C. and fired at 500° C. in flowing air.

The catalyst composition prepared in accordance with this Example 1 hadthe palladium split approximately 62% on the stabilised alumina and 38%on the stabilised ceria.

In a modification of this Example 1, slurry B could be prepared byco-milling NiO and Zr-stabilised ceria as opposed to pre-milling the NiOand then adding the Zr-stabilised ceria.

EXAMPLE 2

An aqueous solution of Pd(NO₃)₂ was impregnated into La-stabilised Al₂O₃with an incipient wetness water absorption of about 1.85 ml/g and thewet powder was dried at 60° C. and fired at 500° C. in static air. Thethus obtained Pd/La-stabilised Al₂O₃ powder was substituted for some orall of the La-Stabilised Al₂O₃ used to make slurry A as in Example 1.Separately, an aqueous solution of Pd(NO₃)₂ was impregnated intoZr-stabilised CeO₂ with an incipient wetness water absorption of about0.5 ml/g and the resulting wet powder was dried at 60° C. and fired at500° C. in static air. The thus obtained Pd/Zr-stabilised CeO₂ wassubstituted for some or all of the Zr-stabilised CeO₂ used to makeslurry B as in Example 1. The rest of the catalyst preparation wasidentical to the method of Example 1 except that the Pd impregnation hadalready been carried out prior to the formation of the slurries.

NOTES:

(i) In a modified method of preparing the catalyst, barium can beincluded in the washcoating step by co-milling a barium compound (egbarium sulphate, barium nitrate or barium acetate) with La-stabilisedAl₂O₃ during the preparation of slurry A. This change would reduceproduction costs by reducing the preparation from three steps to twosteps.

(ii) Nitrates, acetates and chlorides are suitable impregnation salts.

(iii) The catalyst compositon may contain non-Pd-containingZr-stabilised ceria and non-Pd-containing La-stabilised alumina.

(iv) Unstabilised ceria and unstabilised alumina can be used.

(v) The catalysts obtained in Examples 1, 2 and 3 above had the samecomposition, namely:

(a) 2.1% Pd (b) 52.5% Zr-stabilised CeO₂ (c) 23.0% La-stabilised Al₂O₃(d) 6.9% Nd₂O₃ (e) 13.4% BaO (f) 2.1% NiO

All percentages are weight percentages.

(vi) Other useful catalyst compositions of the invention are as follows:

(A) (B) Zr-stabilised CeO₂ 55.8% 51.8% La-stabilised Al₂O₃ 24.4% 22.7%Nd₂O₃ 3.1% 6.8% BaO 10.9% 13.3% NaO 3.1% 2.9% Pd 2.3% 1.0% Rh — 0.1%Washcoat load 0.13 g/cm³ 0.145 g/cm³

Again, all percentages are weight percentages.

(vii) As with all catalyst systems the precious metal content of thecatalyst can vary widely. Also similar composition ranges with washcoatloads up to 0.274 g/cm³ have been tested and found to be successful.

EXAMPLE 3 Comparative Example

This Example is directed to a comparative catalyst, not in accordancewith the present invention, in which soluble ceria is present in thewashcoat.

Cerium carbonate was slurried in water. A 10% excess amount of glacialacetic acid was added to this slurry to completely convert the ceriumcarbonate to cerium acetate. Zirconium acetate was then added to theaforementioned mixture. Finally, La-stabilised alumina and NiO wereadded to form a slurry with approximately 50% solids. The slurry wasmilled to a mean particle size of about 5 microns and coated on a smoothmonolith substrate by dipping (or alternatively passing through awashcoat curtain). After blowing off the excess washcoat with compressedair, the coated substrate was then dried at 60° C. and fired at 500° C.in flowing air. This process may have to be repeated to achieve thedesired washcoat loading. The fired catalyst was dipped in an aqueoussolution of Pd(NO₃)₂/citric acid/Nd(NO₃)₂ (or alternatively it can bepassed through a washcoat curtain of the same solution) and the excesssolutions was blown off with compressed air (Impregnation 1). Thisimpregnation solution was absorbed such that the quantity of solutionwhich just filled the pores of the washcoat contained sufficient Pd andNd to give the desired loadings. The resulting impregnated block wasdried at 60° C. and fired at 500° C. in flowing air. Finally, the firedblock was dipped in an aqueous solution of barium acetate (oralternatively it can be passed through a curtain of the same solution)and the excess solution was blown off with compressed air (Impregnation2). The solution was absorbed such that the quantity of solution whichjust filled the pores of the washcoat contained sufficient barium togive the desired loading. The impregnated block was dried at 60° C. andfired at 500° C. in flowing air.

EXAMPLE 4 Test Results

Laboratory tests of the catalysts of Examples 1, 2 and 3 were conductedin the following manner.

A cylindrical core of 2.54 cm diameter and 30 mm length was cut fromeach of the impregnated blocks of Examples 1, 2 and 3. Each core wasplaced in an oven which had a controlled atmosphere capable of cyclingbetween 1% CO/10% H₂O/20 ppm SO₂/balance N₂ and 0.5% O₂/10% H₂O/20 ppmSO₂/balance N₂ every five minutes. The furnace was heated to 1050° C.and held at that temperature for 12 hours. Each core was removed andtested in a laboratory reactor under lean/rich cycled conditions withthe following average simulated exhaust gas composition:

200 ppm C₃H₈ 200 ppm C₃H₆  1% CO 2000 ppm  NO    0.33% H₂    0.755% O₂14% CO₂ 10% H₂O balance N₂

with a total flow rate of 23.8 SLPM. After establishing this inlet gascomposition, the inlet gas temperature was raised to 550° C. and thetotal HC, CO and NO× percentage conversions were measured. Thetemperature was then lowered to 450° C. and the conversions weremeasured again, and finally the temperature was lowered to 350° C. andthe conversions were measured yet again.

TABLE 550° C. 450° C. 350° C. Catalyst HC CO NO_(x) HC CO NO_(x) HC CONO_(x) Example 1 72 79 63 68 75 58 46 53 35 Example 2 66 87 59 59 71 4741 49 31 Example 3 63 68 52 59 61 45 38 41 28

What is claimed is:
 1. A method of making a platinum group metalthree-way catalyst composition which comprises a high temperaturecatalytic component, a low temperature catalytic component, and anoxygen storage material wherein each catalytic component is present inthe catalyst composition as separate distinct particles in a singlelayer and the oxygen storage material comprises a particulate supportmaterial for the high temperature catalytic component, and wherein thehigh temperature catalytic component exhibits enhanced catalyticactivity at temperatures above 500° C. and the low temperature catalyticcomponent exhibits enhanced catalytic activity at temperatures in therange of 200 to 400° C., the method comprising: (a) forming on anon-porous substrate a combined washcoat of a high temperature catalystsupport material and a low temperature catalyst support material from aslurry in which each of the catalyst support materials is ofsufficiently large particle size so as to prevent each catalyst supportmaterial from forming a solution or a sol with the liquid medium of theslurry, wherein the high temperature catalyst support material comprisesthe oxygen storage material which comprises the particulate supportmaterial; and (b) impregnating a platinum group metal or metals intoeach catalyst support material either by adding the platinum group metalor metals to the slurry of the high temperature catalyst supportmaterial and the low temperature catalyst support material beforeforming the combined washcoat on the non-porous substrate or by applyingthe platinum group metal or metals to the washcoat after formation ofthe washcoat on the non-porous substrate, wherein the high temperaturecatalytic component comprises a first platinum group metal or metalsthat is supported on the oxygen storage material and the low temperaturecatalytic component comprises a second platinum group metal or metalsthat is supported on the low temperature catalytic support material,wherein the second platinum group metal or metals is the same as ordifferent from the first platinum group metal or metals and wherein theamounts of high temperature and low temperature catalyst supportmaterials present in the catalyst composition are determined by theamount of platinum group metal or metals required in each supportmaterial and the ratio of the high temperature catalyst support materialto the low temperature catalyst support material is determined by therespective incipient water absorption capability of each catalystsupport material wherein the water absorption capabilities of the hightemperature catalyst support material and the low temperature catalystsupport material are respectively 0.2 to 1.0 ml/g and 0.5 to 2.5 ml/g.2. A method as claimed in claim 1 wherein the catalyst support materialshave a mean particle size of less than 20 microns.
 3. A method asclaimed in claim 2 wherein the mean particle size of the catalystsupport materials is about 5 microns.
 4. A method as claimed in claim 1wherein separate slurries of the high temperature support material andthe low temperature support material are prepared and the two slurriesare then blended together and coated onto the non-porous substrate.
 5. Amethod as claimed in claim 1 wherein the first and second platinum groupmetal or metals are selected from the group consisting of platinum,palladium, rhodium, iridium and combinations thereof.
 6. A method asclaimed in claim 1 wherein the oxygen storage material is selected fromthe group consisting of ceria, a perovskite, NiO, MnO₂ and Pr₂O₃.
 7. Amethod as claimed in claim 1 wherein the oxygen storage material isstabilized with a stabilizer selected from the group consisting ofzirconium, lanthanum, alumina, yttrium, praseodymium and neodymium.
 8. Amethod as claimed in claim 7 wherein the oxygen storage materialcomprises ceria and the stabilizer comprises zirconium.
 9. A method asclaimed in claim 8 wherein the zirconium stabilized ceria contains 2 to50% by weight of zirconium oxide.
 10. A method as claimed in claim 9wherein the zirconium stabilized ceria has a composition of about 58 %by weight of CeO₂ and about 42 % by weight of ZrO₂.
 11. A methodaccording to claim 1 wherein the low temperature catalyst supportmaterial is one of a stabilized and unstabilized alumina.
 12. A methodas claimed in claim 11 wherein the low temperature catalyst supportmaterial is stabilized alumina and the stabilizer for the alumina isselected from the group consisting of lanthanum, barium and zirconium.13. A method as claimed in claim 12 wherein the stabilizer is lanthanmunand the lanthanum stabilized alumina contains 2 to 7% by weight oflanthanum oxide.
 14. A method as claimed in claim 1 which utilizes acatalyst promoter.
 15. A method as claimed in claim 14 wherein thecatalyst promoter is selected from the group consisting of neodymium,barium, cerium, lanthanum, praseodymium, magnesium, calcium andstrontium.
 16. A method as claimed in claim 1 which utilizes a compoundeffective for the suppression of hydrogen sulfide emissions from thecatalyst composition which compound is selected from the groupconsisting of NiO, Fe₂O₃, CaO and BaO.
 17. A method as claimed in claim1 which utilizes a compound which is effective in preventingpreferential absorption of the platinum group metal in one or other ofthe high temperature or low temperature catalyst support materials whichcompound is selected from the group consisting of citric acid, aceticacid and oxalic acid.
 18. A platinum group metal three-way catalystcomposition prepared by the method of claim
 1. 19. A method as claimedin the claim 1, wherein the impregnating step is carried out by addingthe platinum group metal or metals to the slurry of the high temperaturecatalyst support material and the low temperature catalyst supportmaterial before forming the combined washcoat on the non-poroussubstrate.
 20. A method as claimed in the claim 1, wherein theimpregnating step is carried out by applying the platinum group metal ormetals to the washcoat after formation of the washcoat on the non-poroussubstrate.
 21. A platinum group metal three-way catalyst compositioncomprising a high temperature catalytic component having enhancedcatalytic activity at temperatures above 500° C., a low temperaturecatalytic component having enhanced catalytic activity at temperaturesin the range of 200 to 400° C., and an oxygen storage material having awater absorption capability of 0.2 to 1.0 ml/g, wherein each catalyticcomponent is present in the catalyst composition as separate distinctparticles in a single layer, wherein said high temperature catalyticcomponent comprises a first platinum group metal or metals that issupported on said oxygen storage material and said low temperaturecatalytic component comprises a second platinum group metal or metalsthat is supported on a low temperature catalytic support material havinga water absorption capability of 0.5 to 2.5 ml/g, and wherein saidsecond platinum group metal or metals is the same as or different fromsaid first platinum group metal or metals, wherein the amounts of saidhigh temperature and low temperature catalyst support materials in saidcatalyst composition are determined by the amount of platinum groupmetal or metals required in each catalyst support material, wherein saidoxygen storage material comprises a particulate support material forsaid high temperature catalytic component.
 22. A catalyst composition asclaimed in claim 21 wherein the high temperature and low temperaturecatalytic components have a mean particle size of less than 20 microns.23. A catalyst composition as claimed in claim 23 wherein the meanparticle size of the catalytic components is about 5 microns.
 24. Acatalyst composition as claimed in claim 21 wherein said oxygen storagematerial is impregnated with said first platinum group metal or metals.25. A catalyst composition as claimed in claim 24 wherein said oxygenstorage material is selected from the group consisting of ceria, aperovskite, NiO, MnO₂ and Pr₂O₃.
 26. A catalyst composition as claimedin claim 25 wherein the oxygen storage material is a stabilized ceria.27. A catalyst composition as claimed in claim 26 wherein the oxygenstorage material is zirconium stabilized ceria.
 28. A catalystcomposition as claimed in claim 27 wherein the zirconium stabilizedceria contains 2 to 50% by weight of zirconium oxide.
 29. A catalystcomposition as claimed in claim 28 wherein the zirconium stabilizedceria has a composition of about 58% by weight CeO₂ and 42% by weightZrO₂.
 30. A catalyst composition as claimed in claim 21 wherein the lowtemperature catalytic component is a stabilized or unstabilized aluminaimpregnated with a platinum group metal or metals.
 31. A catalystcomposition as claimed in claim 32 wherein the stabilizer for thealumina is selected from the group consisting of lanthanum, barium andzirconium.
 32. A catalyst composition as claimed in claim 31 wherein thelanthanum stabilized alumina contains 2 to 7% by weight lanthanum oxide.33. A catalyst composition as claimed in claim 21 wherein the platinumgroup metal is selected from the group consisting of platinum,palladium, rhodium, ruthenium, iridium, and any combination thereof. 34.A catalyst composition as claimed in claim 21 further comprising acatalyst promoter.
 35. A catalyst composition as claimed in claim 34wherein said catalyst promoter is selected from the group consisting ofneodymium, barium, cerium, lanthanum, praseodymium, magnesium, calciumand strontium.
 36. A catalyst composition as claimed in claim 21 furthercomprising a compound effective for the suppression of emissions ofhydrogen sulfide from said catalyst composition wherein said compound isselected from the group consisting of NiO, Fe₂O₃, CaO and BaO.
 37. Anengine whose exhaust apparatus contains the catalyst defined in claim21.
 38. A catalyst composition according to claim 21, further comprisinga non-porous substrate on which the washcoat layer is formed.
 39. Acatalyst composition according to claim 21, further comprising anon-porous substrate on which the washcoat layer is formed.
 40. A methodof reducing levels of hydrocarbons, carbon monoxide, and nitrogen oxidesin an exhaust gas from an internal combustion engine comprisingcontacting the exhaust gas with a platinum group metal three-waycatalyst composition comprising a high temperature catalytic componenthaving enhanced catalytic activity at temperatures above 500° C., a lowtemperature catalytic component having enhanced catalytic activity attemperatures in the range of 200 to 400° C., and an oxygen storagematerial having a water absorption capability of 0.2 to 1.0 ml/g,wherein each catalytic component is present in the catalyst compositionas separate distinct particles in a single layer, wherein said hightemperature catalytic component comprises a first platinum group metalor metals that is supported on said oxygen storage material and said lowtemperature catalytic component comprises a second platinum group metalor metals that is supported on a low temperature catalytic supportmaterial having a water absorption capability of 0.5 to 2.5 ml/g, andwherein said second platinum group metal or metals is the same as ordifferent from said first platinum group metal or metals, wherein theamounts of said oxygen storage material and said low temperaturecatalyst support material in the catalyst composition are determined bythe amount of platinum group metal or metals required in each catalystsupport material, wherein said oxygen storage material comprises aparticulate support material for said high temperature catalyticcomponent.
 41. The method as claimed in claim 40 wherein the hightemperature and low temperature catalytic components have a meanparticle size of less than 20 microns.
 42. The method as claimed inclaim 41 wherein the mean particle size of the catalytic components isabout 5 microns.
 43. The method as claimed in claim 40 wherein saidoxygen storage material is impregnated with said first platinum groupmetal or metals.
 44. The method as claimed in claim 43 wherein saidoxygen storage material is selected from the group consisting of ceria,a perovskite, NiO, MnO₂ and Pr₂O₃.
 45. The method as claimed in claim 44wherein the oxygen storage material is a stabilized ceria.
 46. Themethod as claimed in claim 45 wherein the oxygen storage material iszirconium stabilized ceria.
 47. The method as claimed in claim 46wherein the zirconium stabilized ceria contains 2 to 50% by weight ofzirconium oxide.
 48. The method as claimed in claim 47 wherein thezirconium stabilized ceria has a composition of about 58 % by weightCeO₂ and 42% by weight ZrO₂.
 49. The method as claimed in claim 40wherein the low temperature catalytic component is a stabilized orunstabilized alumina impregnated with a platinum group metal or metals.50. The method as claimed in claim 49 wherein the stabilizer for thealumina is selected from the group consisting of lanthanum, barium andzirconium.
 51. The method as claimed in claim 49 wherein the lanthanumstabilized alumina contains 2 to 7% by weight lanthanum oxide.
 52. Themethod as claimed in claim 40 wherein the platinum group metal isselected from platinum, palladium, rhodium, ruthenium, iridium, and anycombination thereof.
 53. The method as claimed in claim 40 furthercomprising a catalyst promoter.
 54. A catalyst composition as claimed inclaim 53 wherein the catalyst promoter is selected from the groupconsisting of Nd, Ba, Ce, La, Pr, Mg, Ca and Sr.
 55. A catalystcomposition as claimed in the claim 40 which contains a compoundeffective for the suppression of emissions of hydrogen sulphide from thegroup consisting of the catalyst composition which compound is selectedfrom NiO, Fe₂O₃, CaO or BaO.
 56. A catalyst composition comprising amaterial impregnated with a first platinum group metal or metals andalumina having a water absorption capability of 0.5 to 2.5 ml/gimpregnated with a second platinum group metal or metals, wherein saidmaterial is selected from the group consisting of ceria, stabilizedceria, a perovskite, NiO, MnO₂ and Pr₂O₃ and has a water absorptioncapability of 0.2 to 1.0 ml/g and said second platinum group metal ormetals is the same as or different from said first platinum group metalor metals, wherein each of said material and said alumina is present insaid catalyst composition as separate distinct particles in a singlelayer, wherein the amounts of said material and said alumina in saidcatalyst composition are determined by the amount of platinum groupmetal or metals present in each of said material and said alumina,wherein said catalyst composition further comprises a catalyst promotercomprising barium.
 57. A catalyst composition as claimed in claim 56wherein said material comprises stabilized ceria and said ceria isstabilized by a stabilizer selected from the group consisting ofzirconium, lanthanum, alumina, yttrium, praseodymium and neodymium. 58.A catalyst composition comprising a material impregnated with a firstplatinum group metal or metals and alumina having a water absorptioncapability of 0.5 to 2.5 ml/g impregnated with a second platinum groupmetal or metals, wherein said material is selected from the groupconsisting of ceria, stabilized ceria, a perovskite, NiO, MnO₂ and Pr₂O₃and has a water absorption capability of 0.2 to 1.0 ml/g and said secondplatinum group metal or metals is the same as or different from saidfirst platinum group metal or metals, wherein each of said material andsaid alumina is present in said catalyst composition as separatedistinct particles in a single layer, wherein the amounts of saidmaterial and said alumina in the catalyst composition are determined bythe amount of platinum group metal or metals present in each of saidmaterial and said alumina, wherein said catalyst composition furthercomprises a catalyst promoter comprising magnesium.
 59. A catalystcomposition as claimed in claim 58 wherein said material comprisesstabilized ceria and said ceria is stabilized by a stabilizer selectedfrom the group consisting of zirconium, lanthanum, alumina, yttrium,praseodymium and neodymium.
 60. A catalyst composition comprising MnO₂having a water absorption capability of 0.2 to 1.0 ml/g impregnated witha first platinum group metal or metals and alumina having a waterabsorption capability of 0.5 to 2.5 ml/g impregnated with a secondplatinum group metal or metals, wherein said second platinum group metalor metals is the same as or different from said first platinum groupmetal or metals and each of said MnO₂ and said alumina is present insaid catalyst composition as separate distinct particles in a singlelayer, wherein the amounts of said MnO₂ and said alumina in saidcatalyst composition are determined by the amount of platinum groupmetal or metals present in each of said MnO₂ and said alumina.